1. Field of the Invention
The present invention relates to energy storage, such as in ultracapacitors and solid-state batteries, and, more specifically, to energy storage with power sources that use acidic electrolytes.
2. Description of the Prior Art
Aerogels (and related ambigels) are sol-gel-derived nanoarchitectures composed of a three-dimensional network of nanoscale particles intermingled with a continuous, aperiodic mesoporosity. The architectural characteristics of high surface area and continuous porosity enhance the transport of ions and molecules throughout the pore-solid architecture for interaction with the nanoscopic solid domains. This combination of properties, which are intrinsic to electrically conductive aerogels, makes them attractive candidates as electrode materials for energy-storage devices including batteries and ultracapacitors. Aerogels and ambigels based on metal oxides, such as manganese oxide (MnO2), are particularly relevant for charge storage, as such oxides undergo reversible cation-electron insertion reactions. Nanostructured metal oxide electrodes exhibit superior performance when used as lithium-battery electrodes or ultracapacitors. See D. R. Rolison & B. Dunn, J. Mater. Chem., 11, 963 (2002), incorporated herein by reference.
Ultracapacitors are a class of energy-storage materials that offer significant promise in bridging the performance gap between the high energy density of batteries and the high power density derived from dielectric capacitors. Currently, high-performance ultracapacitors are based on nanoscale forms of mixed ion-electron conducting metal oxides, such as RuO2, which store charge via a cation-electron insertion mechanism.RuIVO2+xe−+xH+HxRuIIIxRuIV1-xO2  [Equation 1]The charge/discharge profiles associated with such reactions often mimic those of capacitors with a constant charge released or stored over a broad potential range, and thus this type of charge storage is often designated as pseudocapacitance. Ultracapacitors based on hydrous RuO2 yield specific capacitances as high as 768 F/g. The application of RuO2 is limited however by its high cost as a platinum-group metal and its non-domestic sources.
The abundance of manganese minerals and the low toxicity of manganese precursors make MnO2 both an economical and an environmentally benign alternative to RuO2. Manganese oxides are well-studied materials for use as insertion electrodes, with applications ranging from alkaline Zn/MnO2 cells (equation 2) to lithium-ion batteries (equation 3).MnIVO2+xe−+xH+HxMnIIIxMnIV1-xO2  [Equation 2]MnIVO2+xe−+xLi+LixMnIIIxMnIV1-xO2  [Equation 3]Manganese oxides may also be synthesized in a wide range of polymorphs, each with characteristic electrochemical properties. Manganese oxides have been investigated as ultracapacitor electrodes in neutral aqueous electrolytes. Specific capacitance values for MnO2 are as high as 700 F/g for thin-film electrodes, although practical MnO2 electrode configurations yield only 200 F/g. See S. C. Pang, M. A. Anderson & T. W. Chapman, J. Electrochem. Soc., 147, 444 (2000); H. Y. Lee & J. B. Goodenough, J. Solid State Chem., 144, 220 (1999); and J. W. Long, A. L. Young & D. R. Rolison, Advanced Batteries and Super Capacitors, G. Nazri, R. Koetz, B. Scrosati, P. A. Moro, E. S. Takeuchi (Eds.) PV 2001-21, Electrochemical Society (Pennington, N.J.), 2003, pp. 752–759, all of which are incorporated herein by reference.
Previous studies with hydrous RuO2 have demonstrated that the maximum ultracapacitance is achieved in acidic electrolytes, where high concentrations of highly mobile protons are available to the oxide electrode. See L. D. Burke, O. J. Murphy, J. F. O'Neill & S. Venkatesan, J. Chem. Soc., Faraday Trans., 73, 1659 (1977) and E. W. Tsai & K. Rajeshwar, Electrochim. Acta, 36, 27 (1991), both of which are incorporated herein by reference. However, manganese oxide undergoes a reductive-dissolution process when exposed to even mildly acidic electrolytes, yielding water-soluble Mn(II) species.MnIVO2+H++e−MnIIIOOH  [Equation 4]2MnIIIOOH+2H+→MnIVO2+MnII(soluble)+2H2O  [Equation 5]Redeposition of MnO2 via electro-oxidation of Mn(II) is inhibited in acid electrolytes, requiring high overpotentials and elevated temperatures to achieve significant deposition rates. The use of MnO2 as an ultracapacitor, therefore, is limited to near-neutral-pH aqueous electrolytes where the pseudocapacitance is restricted by the presence of less-desirable insertion cations, such as Li+ and K+, which compete with H+ for association at the MnO2 electrode.
Conducting polymers are also being investigated as ultracapacitors because of their ability to undergo electrochemically driven ion-insertion reactions. See A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld & J. P Ferraris, J. Power Sources, 47, 89 (1994), incorporated herein by reference. The energy density of ultracapacitors based on conducting polymers is restricted by the low mass-density of the active organic component as well as the low ion-doping levels, typically less than 0.5 electrons/ions per monomer unit. This limitation of conducting polymer ultracapacitors can be somewhat offset by pairing p-doped and n-doped polymer electrodes in nonaqueous electrolytes, where higher cell voltages (2–3 volts) can be achieved. However, these electrolytes have the further disadvantages of cost and flammability relative to aqueous acid electrolytes.